This invention relates to improved residual petroleum fuel oil compositions and to a method of preparing the same. More particularly, this invention deals with the control of dispersed sedimentary asphaltic constituents, such as asphaltenes and carbenes that can precipitate from residual fuel oils and is particularly concerned with the stabilization of intermediate fuels which are blends of distillate and residual fractions from crude processing.
Various types of instability may be exhibited by residual fuel oils. Among these are: (1) separation of asphaltic or carbonaceous matter, sludge, dirt and water during storage at normal temperatures; (2) separation of black waxy material during storage at low temperatures; (3) increase in viscosity during storage at normal temperatures; and (4) incompatibility or separation of insoluble matter on mixing of fuel oils from different sources. Although the commercially available fuel oils may vary widely in their tendency toward any of the above types of instability all may show some evidence of such instability.
Most present-day residual and intermediate fuel oils contain heavy asphaltic stocks in widely varying proportions. There is some evidence that certain constituents of these asphaltic stocks such as asphaltenes, carbenes, and the like are colloidal in nature and thus blends containing such stocks would not be expected to form true solutions in all cases. Rather, some constituents would be dispersed in the blend and might separate under certain conditions of storage and use.
In the past, the precipitation of asphaltenes and resins from residual, i.e. residuum containing, fuels has been largely avoided by proper selection of blending components. Only distillate and residuum from the same or similar crudes were mixed so there was less likelihood of colloid destruction through changes in solvency. In addition, the severity of reduced crude processing (cracking, distillation, desulfurizing) was controlled to a level that produced distillate and residuum which, on reblending, provided compatible fuels. However, as crude availability tightened due to depletion of reserves and changes in political climate, and also as the need increased to process certain component fractions more severely to reduce sulfur levels, the refiner lost flexibility. It became increasingly difficult to make components that would ensure compatible blends, particularly those also meeting low sulfur specifications.
On occasion, fuel blends are prepared in refineries that inadvertently form precipitates in excess of specification. Ways must then be found to dispose of these blends, such as by "blending-off", reprocessing or post treatment with an additive that will resuspend the material that has precipitated in a form that will not clog the filters, nozzles, etc., of a combustion system.
Additives of the detergent or dispersant type that are added to hydrocarbon fuels to control sludge separation are sometimes claimed to stabilize fuels against asphaltic constituent separation. However, most of them are either ineffective or only marginally effective at practical treating levels, especially for `low sulfur` intermediate fuels. Structurally, these additives are usually metal salts of alkylarylsulfonic acids (see U.S. Pat. No. 2,888,338) or complex ashless dispersants containing amine, imide, ester, or hydroxyl type polar functionality attached to an oil-soluble hydrocarbon chain (see Canadian Pat. No. 605,449 and U.S. Pat. No. 2,958,590).
Oil-soluble sulfonate additives have been taught to be useful for stabilization against oxidative deterioration (not sedimentation of asphaltic constituents) of middle distillate petroleum fuel oil compositions (see Canadian Pat. No. 607,389 and U.S. Pat. No. 2,923,611).
Precipitation of asphaltenes is most likely to occur when the blended fuel is not sufficiently aromatic or naphthenic to provide adequate solvency. The tendency towards separation, therefore, increases with paraffinicity which is particularly serious with low sulfur fuels, where the residual component is frequently only 5-15% of the blend and the distillate has been hydrogen treated to remove sulfur or derived from a low sulfur paraffinic crude, for such blended residual fuels, i.e. intermediate fuels, are very susceptible to colloid degradation and asphaltene sedimentation.
Having briefly described the asphaltene sediment formation problems of residual fuels, it is an object of this invention to afford compositions of this type that are particularly adapted to overcome and avoid these problems.